Electrical cables used in the newer and deeper oil wells to power such things as pumps are subject to high pressures due to the static head of liquid in the well. These pressures can be on the order of about 3,000 psi. These liquids are typically oil and water and are carriers of dissolved gases such as carbon dioxide, methane and hydrogen sulfide. After a cable is inserted into an oil well, these gases tend to migrate through the outer protective sheath into the permeable cable insulation, which is typically formed of rubber. The rate of migration of these gases into the insulation is slowed by the outer sheath which is usually formed of thermoplastic or metallic material and therefore has a low gas-permeability.
However, after a period of time, gases will pass through the outer sheath and saturate the rubber insulation. This process of "loading" of the cable with gases may take many months as the gases slowly migrate into the cable through the outer sheath or through various splices where the chemical barrier formed by the sheath has been removed.
While the cable is located in the well, these gases do not present a significant problem. However, when the cable is suddenly removed from the well or the static head of liquid is removed by a rapid "pump down", the absorbed gases in the insulation will try to escape due to a rapid decompression as the outer pressure on the cable reduces.
Since the outer sheath has a low gas-permeability, these gases cannot pass therethrough rapidly. Thus, the rapid decompression of the cable can cause an explosion, resulting in severe damage to the cable, or a rushing of gases through the splice, resulting in splice damage.
Conventional cables have not encountered this problem since conventional wells are not very deep and there is no significant decompression upon removing the cable or pumping down the well. Thus, conventional armor sheaths can withstand the typical decompressive pressures.
However, with the ever increasing depth of new oil wells, significant decompressive pressure changes can result.